Anhydrous chromic acid metal treating solution



United States 3,437,531 ANHYDROUS CHROMTC ACID METAL TREATING SOLUTEON Rudolf E. Svadlenak, Lewiston, N.Y., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del, a corporation of Delaware No Drawing. Filed July 26, 1962, Ser. No. 212,712 Int. Cl. C23f 7/26 US. Cl. 148-62 7 Claims This invention relates to a new and improved metal treatment solution and process for the treatment of metals therewith. More particularly, this invention relates to an anhydrous chromic acid metal treating solution and a process of treating aluminum, aluminum alloy, zinc, iron and steel articles to impart thereto a higher resistance to corrosion and an improved bonding of paints, lacquers and the like thereto.

The treatment of metal surfaces with chromic acid in aqueous solution is known in the art. Such processes have not been satisfactory since they involve numerous steps and have not imparted a satisfactory corrosion resistance nor satisfactory paint adhesion to the metal surfaces. Moreover, aqueous chromic acid solution treatments required a drying operation prior to painting.

It is an object of this invention to provide a new and improved anhydrous chromic acid metal treating solution.

It is another object to provide a new and improved process for the treatment of aluminum, aluminum alloy, iron and steel structures or articles to impart thereto improved corrosion resistance and paint adhesion.

Other objects of the invention will appear hereinafter.

The objects of this invention may be accomplished, in general, by the provision of a metal treatment solution comprising a chlorinated hydrocarbon solvent containing dissolved therein chromic acid, a zinc compound taken from the group consisting of zinc fluoride, zinc chromate and zinc oxide and a tertiary alcohol soluble in the chlorinated hydrocarbon and containing 4 to 20 carbon atoms, preferably tertiary butanol.

An aluminum, aluminum alloy, iron or steel structure, particularly iron or aluminum strip, may be treated by the steps of immersing the structure in the treating solution until a chromium containing coating is formed thereon and then subjecting the coated surface to ultraviolet light.

By the term chromic acid as used herein is meant chromic anhydride, chromium trioxide, CrO or the hypothetical substance H CrO Reference to quantities of chromic acid are expressed in terms of the equivalent anhydride (CrO By the term aluminum alloy is meant one of the well-known series of alloys containing not less than about 90% aluminum and minor amounts of alloying ingredients such as Cu, Mn, Mg, Si, Zn or Cr. For example, alloy 2021 is composed of 4.5% Cu, 0.6% Mn, 1.5% Mg and the balance Al. Alloy 6061 is composed of 0.25 Cu, 0.6% Si, 1% Mn, 0.25% Cr and the balance Al. Alloy 7075 is composed of 1.6% Cu, 2.5% Mg, 5.6% Zn, 0.3% Cr and the balance Al. Alloy 3003 is composed of 1.2% Mn and the balance aluminum.

In the preparation of the anhydrous chromic acid solution of this invention, the base of the solution is comprised of a chlorinated hydrocarbon solvent. As specific examples of solvents suitable for use in the preparation of the chromic acid solutions are trichlorethylene, perchlorethylene, carbon tetrachloride, chloroform and methylene chloride. Depending upon the mode of operation, the chromic acid is added to the solvent in an amount of between about 0.02% by weight up to the saturation point thereof or even in excess of the saturation point. The chromic acid is dissolved in the solvent 3,437,531 Patented Apr. 8, 1969 by the presence in the solution of tertiary butanol, or other tertiary alcohol, soluble in the solvent, containing 4 to 20 carbon atoms, as a solubilizing agent. The tertiary alcohol should be present in an amount of at least 2% by weight and it may be present in an amount up to 15% or higher, the only limit being the objectionable flammability of the solution if present in too great an. amount. The greater the quantity of tertiary alcohol present, the greater the quantity of CrO that will dis solve in the solution since CrO appears to react with the alcohol as follows:

Of the tertiary alcohols, tertiary butanol appears to be outstanding as a solubilizing agent being operative for extended periods of time. Other tertiary alcohols, soluble in chlorinated hydrocarbon solvents, that may be used include tertiary amyl alcohol; l,l-dirnethyl 2,2-dichloropropanol; 1,l,2,2-tetramethyl propanol and triphenyl carbinol.

Zinc fluoride, zinc chromate or zinc oxide are added as stabilizers. For example, chromic acid in the solution, containing between about 0.02% up to its saturation point in the solution of ZnF ZnCrO, or ZnO, will be 50% reduced after about 30 hours with the solvent at reflux temperature. A similar chromic acid solution containing no zinc fluoride, zinc chromate or zinc oxide will lose 50% of the chloric acid in only 15 hours.

The preferred process of treating the metals is carried out by the following steps:

First, it is preferred, although not essential, to brush the metal with a wire brush or sand blast or otherwise abrade the surface thereof.

The metal, aluminum, aluminum alloy, iron or steel is then degreased by a conventional degreasing process with trichlorethylene or perchlorethylene, i.e., by im- Imersing the same in trichlorethylene or perchlorethylene liquid or vapor, or both, until the metal is clean. This, however, is purely a cleaning step and if the metal is clean can be omitted.

The metal is then immersed in the anhydrous chromic acid treating solution. Although the treating solution may be at any temperature between room temperature and the boiling point of the solvent, it is preferred for economical reasons to operate the treating step at reflux temperature of the solution. The solution is heated at its boiling point and the vapors are condensed by cooling coils surrounding the top inner surface of the treating vessel. Trichlorethylene, methylene chloride and perchlorethylene are therefore eminently suitable as the base solvent in the solution.

The residence time of the metal in the solution should be between about 1 second and 5 minutes depending upon the thickness of the coating desired. Under some circumstances it may be desirable to immerse the metal in the treating solution repeatedly. When the metal is removed from the treating solution operating under reflux conditions, it will necessarily pass through the vapor zone of the solvent. In order to prevent washing off the coating formed on the metal in the liquid solvent solution, it should be passed through the vapor zone quickly to prevent undue condensation of the vapor on the metal.

The chromium oxide coating formed on the metal is a thin powdery layer and does not at this point withstand extensive handling. The coated material is therefore subjected to a treatment with ultraviolet light. Any source of ultraviolet light can be used, for example, an ultraviolet light-emitting sun lamp. The coated surface should remain in the presence of the ultraviolet light for a period of l to 300- seconds depending upon the source of the ultraviolet light and the spacing of the light source from the surface. Thirty seconds exposure at a spacing of 1 inch with Hanovia type SH bulb sold by Englehard Hanovia, Inc., Newark, N.J., will produce the desired setting of the coating to permit handling.

The exposure of the chromium oxide coated surface to ultraviolet light as above described functions to fix the chromium oxide film on the metal surface causing it to become substantially integral with the surface. As an example, two sets of samples of an aluminum panel were treated by first brushing the panels with a wire brush to uniformly abrade the surface and then degreasing the same with trichlorethylene, then treating the same in a solution comprising trichlorethylene containing 0.35% CrO 0.1% ZnF and 5% tertiary butanol for a period of seconds. One set of samples was rinsed with water before ultraviolet light exposure and the other set was first exposed to ultraviolet light from a Hanovia type SH bulb for 30 seconds from a spacing of 1 inch before rinsing with water. All samples .were then exposed for one week in a 5% salt fog cabinet held at 94 F. At the end of this time the samples which had not been first exposed to ultraviolet light were corroded and pitted, whereas the samples that had first been exposed to ultraviolet light showed no signs of corrosion. Similar good results were obtained with the above-described chromic acid solution and ultraviolet light treatment on panels of aluminum alloys 2021, 6061 and 7075.

The following examples illustrate the comparative paint adhesion obtained by the indicated treatment of a series of aluminum panels which were subsequently sprayed and baked to a pencil hardness of HB to F with an alkyd baking enamel having approximately the following formulation:

Percent Pigment 31.2 Urea-formaldehydeabutyl alcohol resin (60% solids) 24.35

55% castor oil modified glyceryl phthalate (60% solids) 30.3 Hydrocarbon solvent 12.25 Candellila wax dispersion solids) 0.2 Tertiary-butyl alcohol 1.7

EXAMPLE I EXAMPLE H A similar series of aluminum and aluminum alloy panels were subjected to brushing with a wire brush until well abraded and degreased with trichlorethylene and then painted and tested in the same manner as in =Example I.

The paint surfaces of all panels were still badly cracked but not as severely as those in Example I.

EXAMPLE III A third series of aluminum and aluminum alloy panels were degreased with trichlorethylene and then placed for 10 seconds in a solution containing trichlorethylene 0.35% CrO 0.04% ZnF and 5% tertiary butanol which was maintained at reflux temperature, and finally placed for 30 seconds directly beneath a 140 Watt type SH Hanovia ultraviolet bulb with a spacing of about 1 inch.

.4 These panels were then painted and tested the same as in Example I. There was comparatively little paint cracking as a result of the tests on any of the panels.

EXAMPLE IV A fourth series of aluminum and aluminum alloy panels were degreased, chromated, painted and subjected to ultraviolet light, and then tested as in Example III except that they were abraded with a steel brush before degreasing. The paint on these panels withstood the tests with very little cracking or other damage. These panels had superior paint bonding than similarly painted and tested panels that were pretreated by the best previously known methods of surface treatment of aluminum panels.

EXAMPLE V A degreased sample of mild steel was immersed 10 seconds in a refluxing trichlorethylene bath containing 0.35% CrO 04% ZnF and 5% tertiary butanol. The sample was removed and exposed to ultraviolet light from a Hanovia type SH bulb for 30 seconds from a distance of 1 inch. The panel was exposed for 10 minutes in a salt fog cabinet. No corrosion was observed on this panel in contrast to severe corrosion on a degreased but nontreated control panel.

EXAMPLE VI The use of zinc oxide Degreased aluminum panels (3003 H14) were treated for 10 seconds in a refluxing solution containing 0.35% CrO 5% t-BuOI-I and trichlorethylene which was saturated with zinc oxide. The panels were exposed to ultraviolet light from a Hanovia type SH bulb for 30 seconds at a distance of 1 inch. The samples were exposed for one week in a 5% salt fog cabinet held at 94 F. At the end of this time the panels were found to be in excellent condition whereas untreated controls were corroded and pitted.

EXAMPLE VII The use of methylene chloride Degreased aluminum panels (3003 H14) were treated for 10 seconds in a refluxing solution containing 0.40% CrO 5.46% t-BuOH, -0.04% ZnF and methylene chloride. The panels were exposed to ultraviolet light from a Hanovia type SH bulb for 30 seconds at a distance of 1 inch. The samples were exposed for one week in a 5% salt fog cabinet held at 94 F. At the end of the time the panels were found to be in excellent condition whereas untreated controls were corroded and pitted.

EXAMPLE VIII The use of perchlorethylene Degreased aluminum panels (3003 H14) were treated for 10 seconds in a refluxing solution containing 0.31% CrO 4.65% tertiary butanol and 0.04% zinc fluoride and perchlorethylene. The panels were exposed to ultraviolet light from a Hanovia type SH bulb for 30 seconds at a distance of 1 inch. The samples were exposed for one week in a 5% salt fog cabinet held at 94 F. At the end of the time the panels were found to be in excellent condition whereas untreated controls were corroded and pitted.

EXAMPLE IX Operation of the process at room temperature Degreased aluminum panels were treated for 10 seconds at room temperature in a solution containing 5.0% tertiary butanol, 0.4% C10 and 94.6% trichlorethylene. The solvent was allowed to volatilize at room temperature and then the panels were exposed to ultraviolet light from a Hanovia type SH bulb for 30 seconds at a distance of 1 inch. The samples were exposed for one week in a 5% salt fog cabinet held at 94 F. At the end of the time the panels were found to be in excellent condition whereas untreated controls were corroded and pitted.

Throughout the specification and claims, any reference to parts, proportions and percentages refers to parts, proportions and percentages by weight unless otherwise specified.

Since it is obvious that many changes and modifications can be made in the above-described details Without departing from the nature and spirit of the invention, it is to be understood that the invention is not to be limited to said details except as set forth in the appended claims.

What I claim is:

1. A metal treating solution consisting essentially of a chlorinated hydrocarbon, chromic acid in an amount sufficient to render a metal base corrosion resistant and a tertiary alcohol in an amount less than 15% by weight and sufiicient to form an ester with said chromic acid.

2. A metal treating solution consisting essentially of a chlorinated hydrocarbon solvent, chromic acid in an amount from about 0.02% by weight up to the saturation point of chromic acid in the solution, and a tertiary alcohol in an amount from that which is stoichiometrically equivalent to the amount of chromic acid up to about 15 by weight.

3. A metal treating solution consisting essentially of a chlorinated hydrocarbon solvent, chromic acid in an amount from about 0.02% by weight up to the saturation point of chromic acid in the solution, and from about 2% to about 15% by weight of a tertiary alcohol which is soluble in the chlorinated hydrocarbon solvent and which contains 4 to 20 carbon atoms.

4. A metal treating solution consisting essentially of trichlorethylene, chromic acid in an amount from about 0.02% by weight up to the saturation point of chromic acid in the solution, and from about 2% to about 15% by weight of a tertiary alcohol which is soluble in the trichlorethylene and which contains 4 to 20 carbon atoms.

5. A metal treating solution consisting essentially of methylene chloride, chromic acid in an amount from about 0.02% by weight up to the saturation point of chromic acid in the solution, and from about 2% to about 15 by weight of a tertiary alcohol which is soluble in the methylene chloride and which contains 4 to 20 carbon atoms.

6. A metal treating solution consisting essentially of chlorinated hydrocarbon solvent, chromic acid in an amount from about 0.02% by weight up to the saturation point of chromic acid in the solution, and from about 2% to about 15% by weight of tertiary butyl alcohol.

7. A metal treating solution consisting essentially of trichlorethylene, chromic acid in an amount from about 0.02% by weight up to the saturation point of chromic acid in the solution and from about 2% to about 15% by weight of tertiary butyl alcohol.

References Cited UNITED STATES PATENTS 2,762,732 9/1956 Somers 1486.2 2,114,151 4/1938 Romig 1486.2 1,194,899 8/1916 'Strippel 11793.3 X 1,542,539 6/1925 Wright 117-93.3 2,992,146 7/1961 Low 148-6.l5 3,100,728 8/1963 Vullo et a1. 148--6.15

FOREIGN PATENTS 891,390 3/ 1962 Great Britain.

RALPH S. KENDALL, Primary Examiner.

US. Cl. X.R. 

1. A METAL TREATING SOLUTION CONSISTING ESSENTIALLY OF A CHLORINATED HYDROCARBON, CHROMIC ACID IN AN AMOUNT SUFFICIENT TO RENDER A METAL BASE CORROSION RESISTANT AND A TERTIARY ALCOHOL IN AN AMOUNT LESS THAN 15% BY WEIGHT AND SUFFICIENT TO FORM AN ESTER WITH SAID CHROMIC ACID. 